When switching from NG enlisted to USAR officer, do I have to do MEPS again if I have a commissioning physical that is less than a year old?

You have two questions really. "Is the commissioning physical still valid?" and "will they accept the commissioning physical?".

The physical is still valid. Commissioning physicals have to be completed within two years of your appointment date (if they are over a year old, but less than two, then you will do an addendum) (Joint Regulation*, para 8-17).

Will the USAR accept it as part of your direct commissioning packet? They SHOULD accept it, but you'll want to double-check with the recruiter.
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* Joint Regulation AR 601–270/OPNAVINST 1100.4C CH–2/AFI 36–2003_IP/MCO 1100.75F/COMDTINST M 1100.2E - Military Entrance Processing Station (MEPS) - https://www.health.mil/Reference-Center/Policies/2011/09/13/Military-Entrance-Processing-Station

I wish I knew let me try to ping some people for you.

CSM Charles Hayden CSM Chuck Stafford LTC Trent Klug

Your physical is still valid since it’s less than two years old. If for some reason they don’t accept it you will just need a new physical, you won’t have to do actually do MEPS again.